1. Field of the Invention
The invention relates to a composite membrane comprising a microporous polymer film as a support and a non-porous gas or vapor-separating coating present on that film.
2. Description of the Related Art
A composite membrane is disclosed in EP-A-181,772. That patent application discloses a method for depositing a layer consisting of a solution of polysiloxane in a suitable solvent onto a porous polysulfone film as a support. The solvent is subsequently removed and the polysiloxane is crosslinked to form a solid coating.
This coating is non-porous, which means that essentially no through-holes are present and no void channels extend from one surface of the coating to the opposite surface, and the composite membrane obtained is suitable as a gas separation membrane.
However, that process of the aforementioned application is suited only for depositing non-porous layers onto supports whose surface pores are small, preferably smaller than 0.1 .mu.m. That is why in these conventional membranes the support exhibits a high resistance to the transfer of gas or vapor. The pores in the surface cannot be chosen any larger, however, inasmuch as the deposited solution would enter the pores and the polysiloxane would partially fill the pores. This would cause the effective thickness, as defined hereafter, of the gas separating coating to increase and the attainable gas flux, as defined hereafter, to decrease.
From EP-A-174,918 it is known to solve the aforementioned problem (of supports with large pores requiring the application of thick coatings) by applying on a porous support first a non-porous intermediate layer with good permeability to gases. A thin, selective coating is subsequently applied to this dense intermediate layer. Although the presence of a suitably chosen intermediate layer need not necessarily reduce the separation factor, as defined hereafter, of the composite membrane to any appreciable extent, it does have a negative effect on the gas permeability and the attainable gas flux of the membrane.